The IEBlog recently reported some improvements
in IE7’s JavaScript engine:

We have heard a lot of requests to improve our Jscript engine, especially now that AJAX
sites are becoming more prevalent on the web. I want you all to know that we have been
listening and have recently made some great fixes to our engine to improve the garbage
collection routine and to reduce unbounded memory growth.

In fact many demos of qooxdoo
run much faster now in IE7 compared to IE6. And they are even faster than in Firefox 1.5 in many cases.
This is a huge jump in performance. Microsoft did not tell about their exact modifications, of course.
Anyway, they have fixed the major problem of large JavaScript-based web applications. This problem,
despite having a catchy name, was mentioned many times before likehere,here andthere:
If you have many objects created, which are simply accessible in the current scope, all
methods and features of JavaScript slow down dramatically.

It would be great if these bugs were fixed so I emailedChris Wilson for comfirmation. He replied:

Yes, it is true we’ve done some dramatic improvements to JavaScript
performance. I wouldn’t say we’ve fixed everything, but we’ve addressed
a bunch of memory leak problems and most significantly, taken a
different strategy for the Jscript garbage collector. This last one
makes a huge difference in many current JavaScript-heavy pages.

Really happy with the improvements they’ve made and hope there’s more to come. Recently been designing a new CV site there’s been very little in the way of specific IE7 fixes. Still have to have an additional css for IE6 and below, but at least that will eventually become a thing of the past.

Although we should all be happy because (a lot of us) are web designers and developers, maybe we shouldn’t be happy because Microsoft is just putting bugs right that should have never existed in the first place!

While it is nice that IE 7 has performance and standards improvements in the JScript engine, it’s catch-up arrival is late in the game. JScript (JavaScript) itself is still a client-side “convenience” script. Your good designers are using less and less JavaScript in favor of server-side scripting in every new design. The future trend, mostly based on a culminating increase in data reliance and security, is to use none, or as little JavaScript as possible. If JavaScript is turned off, your web site and your forms better still work without it.

javascript in IE7 RC1 still does not work well for me. Actualy, some javascript that can be executed with the IE7 beta 2 does not work in IE7 RC1.
One of my major problems with the IE& is that I still cannot use properly an old MSDN Library for Visual Studio 6.0 since many links in it execute some javascript which can’t be run with IE7.

As Peter stated I’ve discovered the same issue with Javascript array sizes. Code that has run fine pre IE7 now fails. I’m curious if a solution is available. I’m in agreement that with the advent and growth of AJAX and it’s dynamic capabilities, imposing limits on Javascript within the browser is a step backwards.